Dynamic sequencing of timeslots in wireless communication systems

ABSTRACT

A plurality of timeslot sequences is generated. Resource codes are then hypothetically assigned to the timeslot sequences. Next, a total effective interference value for each timeslot sequence is measured. Resource codes are then actually assigned to the timeslot sequence with the lowest measured total effective interference value.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.10/747,747, filed Dec. 29, 2003, which in turn claims priority from U.S.provisional application No. 60/458,069 filed on Mar. 26, 2003, which areincorporated by reference as if fully set forth.

FIELD OF INVENTION

The present invention relates to wireless communication systems. Moreparticularly, the present invention relates to assigning resources inwireless communication systems.

BACKGROUND

Wireless communication systems generally divide the time axis intocontinuing intervals of equal duration called frames. As shown in FIG.1, a frame 100 is divided into a finite number (Nt) of intervals ofequal duration called timeslots. A particular base station (ortransceiver in the case of a sectored deployment) may use some or all ofthe timeslots for uplink or downlink transmissions as defined by thebase station's timeslot assignment (i.e. the timeslots within each framethat have been allocated for uplink or downlink user traffic). In eachtimeslot, a finite number of codes (Nc) may be assigned fortransmission/reception of voice and/or data (hereinafter “calls”). Thetimeslot(s) and code(s) assigned for a particular call (either in thedownlink or the uplink) may be referred to as the physical channel(s) onwhich the call is being carried.

When a new call is initiated, a radio resource management (RRM) devicedetermines the timeslot(s) and the number of codes in each timeslot thatwill be assigned to the new call. Typically, codes and timeslots areassigned either sequentially or at random. That is, referring now toFIG. 2, assume based on interference (e.g. which timeslots are beingused for user traffic in neighboring cells) and/or traffic volumeconsiderations that four particular timeslots 202, 204, 206, 208 withina particular frame have been allocated for handling uplink traffic. Itis noted that although the four timeslots are shown for simplicity asbeing adjacent to each other, this is of course not necessary.

Where resources are assigned sequentially, the first code of a new callis assigned to the first timeslot 202. Additional codes (of the new calland subsequent calls where possible) are also assigned to timeslot 202until no more codes may be assigned to timeslot 202 because, forinstance, adding any more codes to timeslot 202 would degrade thesignal-to-noise ratio or violate the maximum allowed transmit powerconstraint in the timeslot 202. Once timeslot 202 is no longer able toaccept additional codes, further codes are assigned to timeslot 204until it can no longer accept any more codes. This pattern continues fortimeslots 206 and 208, as needed.

Where resources are assigned randomly, timeslots and codes are simplychosen at random. That is, in FIG. 2, a new call may have any of itscodes assigned to any of the four timeslots assuming the conditions inthe timeslots are sufficient for acceptance of codes.

Neither sequential nor random timeslot assignment is an efficient use ofthe timeslots that have been allocated for uplink and downlink usertraffic because they fail to consider the conditions in the allocatedtimeslots. Therefore, it is desirable to have a method and systemwithout such limitations.

SUMMARY

The present invention relates to a method and apparatus for assigningsystem resources in wireless communication systems. A plurality oftimeslot sequences are generated. Resource codes are then hypotheticallyassigned to the timeslot sequences. Next, a total effective interferencevalue for each timeslot sequence is measured. Resource codes are thenactually assigned to the timeslot sequence with the lowest measuredtotal effective interference value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a frame having a plurality of timeslots wherein each timeslota plurality of codes may be assigned.

FIG. 2 is a plurality of timeslots allocated for handling user traffic.

FIG. 3 is a method for generating a plurality of timeslot sequences forassigning codes so that system resources are optimized.

FIG. 4 is a plurality of timeslots wherein a Figure of Merit has beencomputed for each timeslot and the timeslots are arranged in a timeslotsequence according to their respective Figures of Merit.

FIG. 5 is a method where a plurality of timeslot sequences are generatedand the timeslot sequence and resources are assigned according to thetimeslot sequence having the lowest total effective interference.

FIG. 6 is a wireless communication system wherein timeslot sequences aregenerated and system resources are assigned to wireless transmit/receiveunits according to the timeslot sequence with the lowest total effectiveinterference.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Hereafter, a wireless transmit/receive unit (WTRU) includes but is notlimited to a user equipment, mobile station, fixed or mobile subscriberunit, pager, or any other type of device capable of operating in awireless environment. When referred to hereafter, a base stationincludes but is not limited to a Node-B, site controller, access pointor any other type of interfacing device in a wireless environment.Further, as mentioned in the Background section, it should be noted thatthe term “call” is used to collectively refer to voice and/or datatransmission/reception.

In wireless communication systems, it is preferable to assign codes ofnew calls to timeslots in such a manner that the collective interference(i.e. the total effective interference) of the assigned codes isminimized. The total effective interference is a function of theinterference caused by each individual assigned code and thefragmentation of the assigned codes making up each call. With respect tointerference, the lower the interference of each code the lower thetotal effective interference. Similarly, with respect to fragmentation,the lower the fragmentation of a group of assigned codes making up aparticular call the lower the total effective interference. That is,again with respect to fragmentation, there is more interferencegenerated where assigned codes making up a particular call aredistributed over, for example, two timeslots as opposed to one.

The total effective interference is given by $\begin{matrix}{I_{et} = {{\left( {\sum\limits_{k = 1}^{K}{{I(k)} \cdot \frac{16}{{SF}(k)}}} \right) \cdot {frag\_ penalty}}(j)}} & {{Equation}\quad 1}\end{matrix}$

-   -   where I(k) is the interference of code k, SF(k) is the spreading        factor of code k, and j is the number of time slots used for the        new call.

As explained above, the total effective interference is based on theinterference of each assigned code in a frame and the manner in whichthose codes are distributed across the timeslots making up that frame.When a new call is initiated therefore, the present invention generatesa plurality of timeslot sequences, hypothetically assigns the codes ofthe new call to the timeslot sequences to measure the total effectiveinterference for each timeslot sequence, and actually assigns the codesto the timeslot sequence yielding the lowest total effectiveinterference. To create a timeslot sequence, timeslots allocated foruser traffic are ranked according to a Figure of Merit (FOM). The FOM ofa particular timeslot, say timeslot i is given byFOMi=−α*ΔIi+β*RU _(usable)(i)  Equation 2

In Equation 2, ΔIi is the difference between the measured interference(in dB) in timeslot i (i.e. I_(i)) and the lowest interference among alltimeslots (i.e. I_(min)) allocated for user traffic in a particulardirection (i.e. uplink or downlink). α is a weighting factor foradjusting the weight given to the interference parameter in calculatinga timeslot's FOM. RU_(usable)(i) is the amount of resource units thatcan be used by a new call in timeslot i. β is a weighting factor foradjusting the weight given to the resource unit parameter in calculatinga timeslot's FOM.

The amount of usable resource units in a particular timeslot is given byRU _(usable)(i)=min(RU _(i), min(M, RU _(max)))  Equation 3

In Equation 3, RU_(i) is the number of resource units that are availablein timeslot i, M is the amount of resource units required by the newcall, and RU_(max) is the maximum amount of resource units that can beused by a new call in timeslot i. It should be noted that RU_(max) istypically limited by the wireless transmit/receive unit (WTRU) used tooriginate the new call. Therefore, RU_(max) is typically given by theamount of codes the originating WTRU is capable of using per timeslot.By way of explanation, a resource unit is the use of one code in onetimeslot at a spreading factor of sixteen. For lower spreading factors,more resource units are considered used. To illustrate for a spreadingfactor 8, two resource units are considered used and for a spreadingfactor of 1, sixteen resource units are considered used.

It is preferable to have a plurality of timeslot sequences from whichthe sequence that finally yields the lowest total effective interferencemay be selected. The number of sequences that are generated are purelyoperator preference. To generate additional sequences, the weightingfactors (α,β) are adjusted thereby resulting in additional new FOMs foreach timeslot and possibly new sequences. Once a desired number oftimeslot sequences are generated, codes are hypothetically assignedthereto resulting in a different total effective interference for eachtimeslot sequence. Finally, the one with the lowest total effectiveinterference is selected and codes are assigned to those timeslots insequential order. Of course, the sequences may be adjusted as codes areassigned because each timeslots FOM may change as a result of codesbeing added thereto.

Referring now to FIG. 3, there is shown a method 300 for generating aplurality of timeslot sequences for assigning codes so that systemresources are optimized. The method 300 may be used in each direction(i.e. uplink and downlink) to generate a plurality of uplink timeslotsequences and a plurality of downlink timeslot sequences.

The method 300 begins with step 302 wherein a first weighting factor (a)is selected for evaluating the interference in each timeslot. Next, instep 304, a second weighting factor (B) is selected for evaluating thenumber of useable resource units in each timeslot. Once the weightingfactors are selected, a FOM is computed for each timeslot allocated foruser traffic in the direction (i.e. uplink or downlink) for which thesequence is being generated (step 306). As explained above, the FOM ofeach timeslot is a function of interference and the number of resourceunits that can be used for the new call.

In step 308, the timeslot sequence is created based on the FOM values.To create the sequence, the timeslots are put in order of decreasingFOM. Therefore, the first timeslot in a timeslot sequence has thehighest FOM of the group and the last timeslot in the sequence has thelowest FOM of the group. In step 310, it is determined whetheradditional timeslot sequences are to be created. If no, the method ends(step 312). In yes, the first (α) and second (β) weighting factors areadjusted and the method 300 cycles back to step 306.

To further illustrate the creation of timeslot sequences, reference isnow made to FIG. 4. By way of example, assume we have created a downlinktimeslot sequence using a particular pair of weighting factors whereinthere are four downlink timeslots 402, 404, 406, 408. As explained inconnection with FIG. 2, the timeslots are shown as being adjacent purelyfor convenience. Further assume that the FOM values of timeslots 402,404, 406, 408 are 3, 21, −2, and 7.1 respectively. In this case, basedon the FOM values, timeslot 404 is the first timeslot in the sequence,timeslot 408 is second, timeslot 402 is third, and timeslot 406 isfourth (i.e. the timeslot sequence is 404, 408, 402, and 406).Therefore, assuming this sequence is the sequence with the lowest totaleffective interference, when new calls are initiated their codes areassigned to timeslots according to the created sequence. That is, codesare assigned to timeslot 404 first until it reaches capacity, then totimeslot 408 until it reaches capacity and so on until timeslot 406reaches capacity.

To illustrate a method by which the timeslot sequence with the lowesttotal effective interference is selected, reference is now made tomethod 500 shown in FIG. 5. The method 500 begins with step 502 whereinvarious sets of weight factors (α, β) are used to produce severaldifferent timeslot sequences as explained above. Then starting with thefirst code of a new call (step 504), assign the code to the timeslotwith the highest FOM in this timeslot sequence (step 506).

Once a code has been assigned to a timeslot, the interference in thattimeslot typically increases. Therefore, once the code is assigned tothe timeslot with the highest FOM in step 506, the interference in thattimeslot is updated in step 508. If there are more codes to assign, themethod proceeds to step 510 and then cycles back to step 506. In step510, the FOM of each timeslot is recomputed and the timeslot sequence isupdated to reflect any changes in the sequence based on the updatedFOMs. If there are no more codes to assign, the method proceeds to step512.

The FOM may be recomputed as shown below, if no code of the new call isassigned to timeslot i,FOM _(i) =−α·ΔI _(i) +β·RU _(usable)(i)  Equation 4

-   -   if at least one code of the new call has been assigned to        timeslot i,        FOM _(i) =−α·ΔI _(i) +β·RU _(usable)(i)+α·Hysteresis,  Equation        5    -   where ΔI_(i) and RU_(usable)(i) take the updated values after        the assignment of code(s) in previous steps. For timeslots where        codes of the new call are already assigned, hysteresis is        considered to favor those time slots. Therefore, a higher        fragmentation penalty will occur only when timeslots unused by        the new call have remarkably lower interference.

In step 512, the total effective interference for this timeslot sequenceis recorded. Then, if there are any more timeslot sequences that havenot yet been checked, the method 500 cycles back to step 504. If thereare no more sequences to check, the timeslot sequence yielding thelowest total effective interference as the timeslot sequence in whichcodes will be assigned (step 516).

Referring now to FIG. 6, there is shown a wireless communication system600 wherein timeslots allocated for user traffic may be arranged in atimeslot sequence so as to optimize the assignment of system resources.The wireless communication system includes at least one radio networkcontroller (RNC) 602, at least one base station 604, and a plurality ofWTRUs 606, 608, 610. The RNC 602 includes a processor 612 configured tocompute a FOM for a plurality of timeslots as explained in method 300 inFIG. 3. That is, the timeslots allocated for user traffic may be, ineach direction, organized into a timeslot sequence that begins with thetimeslot having the highest FOM and continues with the remainingtimeslots in order of decreasing FOM. As explained above, processor 612preferably generates a plurality of such timeslot sequences by adjustingthe weighting factors used in computing the FOM.

Processor 612, or another processor 614, is configured to compute thetotal effective interference for each timeslot sequence as explained inmethod 500 of FIG. 5 and assign codes from new calls to the timeslotsequence having the lowest total effective interference. Processors 612,614 may be located within a radio resource management (RRM) device 620within radio network controller 602. The functionality of processors612, 614 may also be performed at the at least one base station 604using processors 616 and/or 618.

It is important to note that the present invention may be implemented inany type of wireless communication system employing any type of timedivision multiple access, such as time division duplex (TDD) technology,as desired. By way of example, the present invention may be implementedin UMTS-TDD, TD-SCDMA, CDMA2000 (EV-DO and EV-DV), or any other type ofwireless communication system. Further, while the present invention hasbeen described in terms of various embodiments, other variations, whichare within the scope of the invention as outlined in the claim belowwill be apparent to those skilled in the art.

1. A method of assigning resources in a wireless communication systemcomprising: (a) generating a plurality of timeslot sequences; (b)assigning resource codes to the timeslot sequences on a hypotheticalbasis for analysis; (c) measuring a total effective interference valuefor each timeslot sequence; and (d) assigning actual resource codes tothe timeslot sequence with the lowest measured total effectiveinterference value.
 2. The method of claim 1 wherein step (a) furthercomprises: (a1) selecting a first weighting factor for evaluatinginterference; (a2) selecting a second weighting factor for evaluatingusable resource units; (a3) computing a figure of merit (FOM) value foreach of a plurality of timeslots allocated for user traffic; (a4)sequencing the timeslots in decreasing FOM-value order wherein atimeslot with the highest FOM value is sequenced first and a timeslotwith the lowest value is sequenced last; and (a5) for each additionalsequence desired to be generated, adjusting the weighting factors ofsteps (a1) and (a2) and repeating steps (a3) and (a4).
 3. The method ofclaim 2 wherein step (b) further comprises: (b1) assigning resourcecodes to a first timeslot in a first timeslot sequence on a hypotheticalbasis until the first timeslot reaches capacity; (b2) assigning resourcecodes to a next timeslot in the first timeslot sequence on ahypothetical basis until the next timeslot reaches capacity; (b3)repeating step (b2) until all timeslots in the first timeslot sequencereach capacity or until resource codes are no longer available; and (b4)repeating steps (b1) through (b3) for all timeslot sequences.
 4. Themethod of claim 2 wherein step (b) further comprises: (b1) assigning afirst resource code on a hypothetical basis to a timeslot with thehighest FOM value in a first timeslot sequence; (b2) updating aninterference value for the highest FOM-value timeslot; (b3) re-computinga FOM value for each timeslot in the first timeslot sequence; (b4)re-sequencing the timeslots in the first timeslot sequence in decreasingFOM-value order; (b5) assigning a next resource code on a hypotheticalbasis to a timeslot with the highest re-computed FOM value in the firsttimeslot sequence; (b6) repeating steps (b2) through (b5) until alltimeslots in the first timeslot sequence reach capacity or untilresource codes are no longer available; and (b7) repeating steps (b1)through (b6) for all timeslot sequences.
 5. The method of claim 4wherein a hysteresis factor is considered in re-computing the FOM valuesof timeslots to which a resource code was hypothetically assigned.
 6. Adevice for assigning resources in a wireless communication systemcomprising: means capable of generating a plurality of timeslotsequences; means capable of assigning resource codes to timeslotsequences on a hypothetical basis for analysis; means capable ofmeasuring a total effective interference value for each timeslotsequence; and means capable of assigning actual resource codes to atimeslot sequence with the lowest measured total effective interferencevalue.
 7. The device of claim 6 further comprising: means capable ofselecting a first weighting factor for evaluating interference; meanscapable of selecting a second weighting factor for evaluating usableresource units; means capable of computing a figure of merit (FOM) valuefor each of a plurality of timeslots allocated for user traffic; meanscapable of sequencing the timeslots in decreasing FOM-value order suchthat a timeslot with the highest FOM value is sequenced first and atimeslot with the lowest value is sequenced last; and means capable ofadjusting the first and second weighting factors.
 8. The device of claim7 further comprising means capable of assigning resource codes to aplurality of timeslot sequences on a hypothetical basis for analysis. 9.The device of claim 8 further comprising means capable of assigningactual resource codes based on said analysis.
 10. The device of claim 6wherein said device is a radio network controller.
 11. The device ofclaim 6 wherein said device is a base station.